1. Field of the Invention
The invention is in the field of photovoltaic devices, such as solar cells.
2. Brief Description of the Background Art
Chalcopyrite semiconductors based on CuInSe2 (or CIS) have been investigated for their application to thin film solar cells for over 25 years. Chalcopyrite semiconductors are formed from group I, group III, and group VI elements of the periodic table. Alloying of CuInSe2 and CuGaSe2 to form Cu(InxGa1-x)Se2 (or CIGS) allows the energy gap to be varied between 1.0 eV and 1.68 eV in order to vary the spectral absorption profile. Alloying with CuInS2 to form Cu(InxGa1-x)(SeyS1-y)2 (or CIGSS) allows band-gaps as high as 2.4 eV to be obtained. These semiconductors are direct gap materials; their high optical absorption coefficient allows absorption of sunlight in layers that are only 2 μm in thickness. The electrical properties of the material are determined by composition, intrinsic defects, and structural defects. The cost of solar energy conversion has already been lowered through use of thin film amorphous silicon photovoltaic modules, and a further cost reduction may be anticipated via the use of high efficiency thin film CIGS devices.
Although CIS materials generally deviate from the exact stoichiometry of CuInSe2, it is found that they are usually described by the pseudo-binary system (1-δ)Cu2Se+(1+δ)In2Se3. Electronically useful films are Cu-poor (Cu/In<1, or more generally, Cu/(In+Ga)<1), and are p-type. All efficient solar cells are made using such p-type material. In Cu-rich films (Cu/(In+Ga)>1) the degenerate semiconductor CuxSe forms, leading to films of a metallic nature. Solar cells made using Cu-rich films are of very poor quality.
A solar cell is essentially a rectifying junction in a semiconductor material in which light can be absorbed. The free electrons and holes generated by the absorption of photons are separated by an internal electric field in the semiconductor, giving rise to a photovoltage. In principle, two types of junction can be envisaged using p-type CIGS. If, for example, a surface layer of the CIGS is made n-type through introduction of n-type dopant atoms, then an n-p homojunction is formed. Alternatively, band-bending in the CIGS can be induced by deposition of an n-type semiconductor material of a completely different composition, thereby forming a heterojunction.
The first solar cells made using CIS as the semiconductor employed a layer of n-type CdS deposited by vacuum evaporation to form what was thought to be a heterojunction. Later, to allow the thickness of the CdS to be reduced, the CdS was overcoated with a transparent conductor (Al-doped ZnO). It was also discovered that the use of CdS layers prepared by chemical bath deposition (CBD) using, for example, an aqueous ammonium hydroxide solution containing cadmium acetate as a Cd source and thiourea as a sulfur source, allowed solar cells of higher conversion efficiency to be produced.
The full structure of this type of thin film cell is ZnO/CdS/CIGS/Mo/glass, where the Mo serves as an ohmic contact at the rear of the device. Usually, the ZnO is deposited as a bi-layer consisting of about 500A (Angstroms) of high resistivity ZnO (i-ZnO) followed by about 4000A of highly conductive ZnO (ZnO:Al). The CdS (together with the i-ZnO) is frequently referred to as a buffer layer that is inserted between the active CIGS layer and the ZnO transparent conductor. The use of a Na-containing glass, e.g. soda-lime glass, is another factor that contributes to the achievement of high efficiencies. In 1999, a record 18.8% total-area conversion efficiency was reported for a CIGS solar cell with this structure (M. A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, and R. Noufi, Prog. Photovolt: Res. Appl. 7, 311-316 (1999)). The CIGS was deposited by a three-stage process based on vacuum evaporation (see U.S. Pat. Nos. 5,441,897 and 5,436,204).
While the use of CBD CdS for junction formation has resulted in the highest conversion efficiencies in the laboratory, its use in high volume manufacturing is problematic owing to the presence of cadmium both in the manufacturing plant and in the product. The generation of large volumes of liquid chemical waste is also a nuisance and a significant cost factor. Consequently, the elimination of the wet CBD CdS step in producing CIGS solar devices and its replacement by a dry process represents an important practical goal.
The attainment of this goal has been sought by researchers around the world and has proven elusive. Omission of the buffer layer usually results in solar cells of very poor efficiency. A study of the literature reveals that over twenty other materials and ten deposition processes have been investigated in the hope of forming satisfactory buffer layers. Materials include ZnS, ZnSe, ZnO, ZnInxSey, Zn(O,S,OH)x, In(OH,S)x, In2Se3, CdSe, CdCl2, Sn(S,O)2, Zn2SnO4, and a-Si:H. Methods include CBD, evaporation, co-evaporation, sputtering, MOVPE (Metal Organic Vapor Phase Epitaxy), MOCVD (Metal Organic Chemical Vapor Deposition), ALE (Atomic Layer Epitaxy), solvent, and PECVD (Plasma Enhanced Chemical Vapor Deposition). No dry process has yet equaled the combination of high cell efficiency and high device yield achievable with CBD CdS.
It appears that CBD CdS confers multiple and distinct benefits. These may well include:                cleaning of the CIGS surface, possibly involving removal of native oxide, and dissolution of sodium carbonate        complete physical coverage of the CIGS        provision of a buffer layer of high, but finite, resistivity, thereby lessening the effect of electrical shunts        in-diffusion of Cd (possibly by Cu—Cd ion exchange) and n-type doping of the surface by Cd donors        provision of a barrier to sputter damage of the CIGS during ZnO deposition        removal of interface acceptors, possibly arising from oxygen on Se sites        improvement of the minority carrier diffusion length in the film bulkThe fact that CBD CdS produces better devices than vacuum evaporated CdS demonstrates that the performance of the buffer layer depends not merely on its chemical composition but on the method of deposition.        
It has been reported that Cd and Zn are n-type dopants in CIS. Since the growth of CBD CdS takes place at the low temperature of about 60-80° C., making it unlikely that true epitaxial growth of CdS on CIS takes place, we assume that the highly efficient CIGS devices are, in fact, homojunctions in which the junction is buried at a small depth inside the CIGS, thereby negating the need for epitaxial growth to ensure a low defect density at the junction interface. We further hypothesize that during CBD, Cd diffuses a short distance into the CIS.